Design rules for advanced generation microlithography (i.e., such as e-beam, X-ray, and extreme ultraviolet (EUV) lithography operating at a wavelength of 13.4 nm) are trending toward ever smaller dimensions of, for example, 30 nm and below. The narrower linewidths and thinner resist films used in advanced generation lithography can give rise to consistency issues such as line width roughness (LWR), where resolution takes on increasing significance and limits the performance and usefulness of photoresists. Excessive LWR can lead to poor etch and lack of linewidth control in, for example, transistor and gate architecture, potentially causing short circuits and signal delay in the final devices.
Uneven distribution of photoacid generators (PAGs) for catalyzing deprotection of protected developable groups in photoresist films may contribute to increased LWR and hence poor resolution. PAGs may be incorporated into photoresist formulations by preparing a physical blend of the PAG and a photoresist polymer where, upon spin coating, an inhomogeneous distribution of PAG may occur in the photoresist film, leading to uneven acid generation and greater line edge roughness (LER). Alternatively, the PAG can be attached to the polymer backbone, limiting its mobility in the formulation. While this strategy can improve PAG dispersion in a photoresist film and hence improve pattern formation, the PAGs nonetheless are doubly distributed between both the polymer chains (where some chains may contain more PAG than others) and within a polymer chain (where some chain regions may contain more PAG than others depending on the reactivity ratio of a PAG-containing monomer). More uniform PAG dispersion methods are therefore desirable.
Control of composition, molecular weight, and polydispersity are thus useful for improving PAG dispersion in a photoresist film. Acrylate-based EUV photoresist polymers may be synthesized by modified free radical polymerization techniques in which control of monomer and initiator feed rates help control the composition, but termination and chain transfer reactions can lead to different compositions occurring at different points during a polymerization, and a relatively broad distribution of molecular weights. Because variation in composition affects photoresist solubility, a broad composition distribution across chains and broad molecular weight distributions are not desirable.
Controlled radical polymerization methods can be used to prepare (meth)acrylate containing polymers with polydispersities of less than 2.0. One method of controlled radical polymerization involves use of dithioester chain transfer agents (CTA) to control molecular weight distributions. As described in Proc. of SPIE, 2008, Vol. 6923, pp. 69232E-1-69232E-9, reversible addition fragmentation transfer (RAFT) polymerization techniques have been used to produce photoresist polymers for 193 nm lithography. Accurate and specific molecular weights may be obtained based on the amount of CTA, and because the CTAs are chain terminators, it is possible to end-group functionalize polymers made using CTAs.